Proxy is built for deleted strip instead of short segment
This would be typical behaviour, as a hard cut trim is a rarely used feature. Great for making freeze frames but not very flexible. More often a user would make a soft cut to allow later alteration, in which case you would want all of the proxy to be available.

Also I imagine that transcoding the entire original media would eliminate the potential for mismatched frames of long GOP (Group of Pictures) sources. Where Blender actually has to decode all of the previous frames to make sure that it creates the correct ones that you have selected to hard cut on.